React will re-render the List immediately after it exits with a return statement. In the above example, setSelection is called directly during a render. Storing information from previous renders like this can be hard to understand, but it’s better than updating the same state in an Effect. You want to reset the selection to null whenever the items prop receives a different array: This List component receives a list of items as a prop, and maintains the selected item in the selection state variable. Sometimes, you might want to reset or adjust a part of the state on a prop change, but not all of it. The fact ProfilePage passes it as a key to the inner Profile component is an implementation detail. Components rendering ProfilePage don’t need to pass the key to it: they pass userId as a regular prop. Note that in this example, only the outer ProfilePage component is exported and visible to other files in the project. Now the comment field will clear out automatically when navigating between profiles. Whenever the key (which you’ve set to userId) changes, React will recreate the DOM and reset the state of the Profile component and all of its children. By passing userId as a key to the Profile component, you’re asking React to treat two Profile components with different userId as two different components that should not share any state. Normally, React preserves the state when the same component is rendered in the same spot. To fix the issue, you want to clear out the comment state variable whenever the userId changes: As a result, it’s easy to accidentally post a comment on a wrong user’s profile. One day, you notice a problem: when you navigate from one profile to another, the comment state does not get reset. The page contains a comment input, and you use a comment state variable to hold its value. This ProfilePage component receives a userId prop. (For example, when Strict Mode is on, you will see each component render twice rather than once.) To get the most accurate timings, build your app for production and test it on a device like your users have. For example, Chrome offers a CPU Throttling option for this.Īlso note that measuring performance in development will not give you the most accurate results. Keep in mind that your machine is probably faster than your users’ so it’s a good idea to test the performance with an artificial slowdown. It only helps you skip unnecessary work on updates. UseMemo won’t make the first render faster. Your first instinct might be to add a fullName state variable and update it in an Effect: Moreover, you’d like fullName to update whenever firstName or lastName change. You want to calculate a fullName from them by concatenating them. Suppose you have a component with two state variables: firstName and lastName. To help you gain the right intuition, let’s look at some common concrete examples! Updating state based on props or state Keep in mind that modern frameworks provide more efficient built-in data fetching mechanisms than writing Effects directly in your components. You can also fetch data with Effects: for example, you can synchronize the search results with the current search query. For example, you can write an Effect that keeps a jQuery widget synchronized with the React state. You do need Effects to synchronize with external systems. This is why you’ll usually handle user events in the corresponding event handlers. By the time an Effect runs, you don’t know what the user did (for example, which button was clicked). In the Buy button click event handler, you know exactly what happened. For example, let’s say you want to send an /api/buy POST request and show a notification when the user buys a product. You don’t need Effects to handle user events.That code will automatically re-run whenever your props or state change. If your Effect also immediately updates the state, this restarts the whole process from scratch! To avoid the unnecessary render passes, transform all the data at the top level of your components. Then React will “commit” these changes to the DOM, updating the screen. When you update the state, React will first call your component functions to calculate what should be on the screen. You might feel tempted to write an Effect that updates a state variable when the list changes. For example, let’s say you want to filter a list before displaying it. You don’t need Effects to transform data for rendering.There are two common cases in which you don’t need Effects: How to notify parent components about changes.Which logic should be moved to event handlers.How to share logic between event handlers.How to reset and adjust component state without Effects.How to cache expensive computations without Effects.Why and how to remove unnecessary Effects from your components.
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